There are a number of different ROADM systems on the market today. Some use wavelength blockers, some use wavelength-selective switches (WSS) and others are based on Planar Lightwave Circuit (PLC) technology. Earlier ROADM configurations were limited to 2-degree nodes, but today wavelength-selective switches have made it possible to design cost-effective, multi-degree ROADM nodes. Multi-degree ROADM's provide a lot of flexibility in a wavelength division multiplexed (WDM) network by allowing arbitrary wavelengths to be added or dropped at a particular node or to be switched from one degree to another. One limitation typically shared by current ROADM products is that they are not “directionless”, i.e. a local add/drop transceiver can only be physically connected to one particular degree (i.e., direction) of the node. If a user wants to switch a particular transceiver to a different degree, the user must manually disconnect the transceiver and reconnect it to a different physical port associated with the new degree.
Directionless ROADMs make it possible for operators to dynamically provision local transceivers to different degrees as they reconfigure their networks. Additionally, directionless ROADMs lay the foundation for network architectures supporting all-optical mesh restoration. One exemplary implementation of a directionless ROADM includes the addition of a photonic cross-connect (PXC) to enable routing of the local add/drop channels to arbitrary degrees on the ROADM.
Referring to FIG. 1, an exemplary 4-degree ROADM 10 is constructed with two add/drop clients, i.e. an Internet Protocol (IP) router/switch 12 and an Optical Transport Network (OTN) switch 14. In a first configuration 20, the ROADM 10 is constrained, in that each add/drop client signal 22, 24 can only route through one degree 30 (i.e., direction). Note that express waves 32 can still pass from any-to-any network fiber, so long as these are not dropped locally. In a second configuration 40, a directionless PXC 42 adds a “switch matrix” to the Add/Drop signal bank, which allows add/drop clients truly to be switched to any degree dynamically. Again, express waves can pass from any-to-any degree.
Directionless ROADMs have been disclosed using large N×N cross-connects. For example, these are disclosed by V. Kaman et al., “Multi-Degree ROADM's with Agile Add-Drop Access,” (available at www.calient.net/_docs/PhotonicSwtConf_MultiDegreeROADMs—07.pdf) and by Sashisekaran Thiagarajan et al., “Direction-Independent Add/Drop Access for Multi-Degree ROADMs”, OFC 2008 Proceedings, OThA7, Optical Society of America, February 2008. Disadvantageously, the problem with large N×N cross-connects is that N scales as the number of degrees multiplied by the number of wavelengths in each degree. For example, a degree 8-node with 80 wavelengths per fiber requires a 640×640 cross-connect which is beyond the current state-of-the art and involves a very high up-front cost even if only a few channels are needed initially, and this would increase to 1280×1280 for 100% add/drop capability. The very large number of interconnects also make it very difficult to install or service the switch fabric and the problem gets worse if a redundant switch fabric is needed for network availability reasons.
Two other approaches to ROADMs are disclosed by Ghelfi et al., “Optical Cross Connects Architecture with per-Node Add & Drop Functionality”, Ghelfi et al., OFC 2007 Proceedings, NTuC3, Optical Society of America, February 2007. First, Ghelfi et al. disclose an optical cross-connect with per-port Add & Drop (OXC-PAD) using a 1×N WSS in each degree to route drop channels to N local transceivers. Add channels are collected using N×1 combiners. Each transceiver is connected to a pair of 1×N switches that direct the Tx/Rx signals to the WSS/combiner module associated with the selected degree. This approach could make sense for a small degree (e.g., less than degree 5) node with a few (e.g., less than 10) add/drop channels, but it becomes prohibitively expensive for larger nodes, as the minimum number of WSS ports required is given by the number of degrees multiplied by the number of add/drop channels. For example, a degree 8-node with 40 add/drop channels requires on the order of forty 1×8 WSSs, for a total of 320 WSS ports. Eighty 1×8 switches are also required.
In a second approach, Ghelfi et al. add an additional degree to the ROADM and connect add/drop local traffic using that degree. Although this is a low-cost approach, it introduces severe blocking issues since it only allows a single add/drop channel to be provisioned at a given wavelength. To enable efficient use of bandwidth in the network, a directionless ROADM needs to be non-blocking, which means that different add/drop channels can be provisioned at the same wavelength if they are connected to different degrees.